Oxygen Adsorption and Diffusion on NiTi Alloy (100) Surface: A

Sep 25, 2012 - Oxygen will bind strongly with the alloy surface and even induce surface ... Shape memory alloy microvalves for a fluidic control syste...
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Oxygen Adsorption and Diffusion on NiTi Alloy (100) Surface: A Theoretical Study Xin Liu,* Huimin Guo,* and Changgong Meng School of Chemistry, Dalian University of Technology, Dalian, 116024, P. R. China ABSTRACT: The formation of the passive oxide surface layer accounts for the superior biocompatibility of NiTi alloy based implant materials. However, the usage NiTi alloy is limited by the long-term release of biotoxic Ni ion from the bulk, facilitated by the formation of defects and vacancies in the surface oxide layer during conventional processing. To aid the improvement of the biocompatibility of NiTi alloys, extensive first-principles based calculations were performed to uncover the microscopic mechanism for the temperature controlled oxidation of NiTi alloy. We show that the oxygen adsorption and diffusion on the NiTi surface are the elementary steps for the formation of a surface oxide layer. Oxygen will bind strongly with the alloy surface and even induce surface reconstruction, and the adsorption energy can be as high as −6.14 eV. The requested surface diffusion for formation of TiO2 surface terminations is thermodynamics driven, but the corresponding kinetics is strongly affected by temperatures. These result in formation of TiO at low temperatures and TiO2 at elevated temperatures.

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INTRODUCTION NiTi alloy has been widely used as an implant material for its good corrosion resistance, superelasticity, and biocompatibility.1−6 The biocompatibility of implants fabricated from NiTi depends highly on the surface titanium oxide layer, which will form well-known benign interactions with the human body.3 Furthermore, Ti metal is also a safe biocompatible material, and the oxide layer also has a dominant contribution in avoiding the release of biotoxic Ni ion from the bulk alloy phase to the human body.6−10 The reaction of oxygen with surface Ti atoms is exothermic, and makes the formation of the surface TiOx layer facile.10−17 Various oxidation treatment protocols, such as conventional oxidation,4,6,11 laser oxidation,17 mechanical polishing,3,7 electropolishing and chemical polishing,7,10 ion beam and electrochemical oxidation, etc.,12 have been designed to fabricate NiTi alloy based implants. The performance of the resulting oxide layer is found strongly dependent on preparation conditions, and the depth-resolved X-ray photoelectron spectroscopy (XPS) results show that the outermost oxide layer has Ti present as Ti4+.6,7,13,17 Inside the oxide layer, Ti4+ species are still present and the concentration of Ti3+ and Ti2+ increases.6,7,14,17 The Ti content in the region right below the TiOx layer is significantly lower and Ni species are enriched, while the NiTi stoichiometry is recovered ongoing into the bulk of the alloy. As the formed TiOx layer can act as a passivation layer, the surface oxidation treatment will not influence the bulk stoichiometry and performance of the NiTi alloy. However, the existence of defects and microchannels in the thick TiOx layer makes the leasing of Ni ion facile and is hard to avoid during conventional handling and further usage.18 Effective processing protocols that can precisely control the surface TiOx formation © 2012 American Chemical Society

and minimize the formation of defects and microchannels should be designed and implemented, but the mechanisms for the TiOx layer formation are seldom explored. Recently, Firstov et al. and Xu et al. found that NiTi alloy exhibits different oxidation behavior at temperatures below and above 773 K. They found that a Ni-free zone was formed in the surface oxide layer when the handling temperature is between 773 and 873 K, and the layer is TiO2 with underneath Ni rich species such as Ni, Ni3Ti, and NiTi. However, at temperatures below 773 K, TiO is the dominant oxide phase at the alloy surface. As the temperature goes above 700 K, Ni oxide and NiTiO3 are also observed, but the layer becomes porous.11,19 This proved a temperature-controlled character of the initial oxide layer formation. The yield of only TiO phase at low oxidation temperature and pure TiO2 phase at high temperatures also reflects the possibility of achieving a precisely controlled oxidation for the high performance TiOx layer.11,20,21 Theoretically, NiTi alloy has been studied to understand the SMA effect and to study the electronic properties of intermetallic alloys.22−28 However, theoretical studies on the oxidation of NiTi alloys are very rare. Kulkova et al. studied the surface electronic structure of low index NiTi alloy surface, and they proposed that the Ti-terminated (100) surface should be highly reactive.25 By using the DVXα method, Liu and Hua et al. studied the electronic interaction between the O2 and NiTi(100) and (110) surfaces. They found that O2 approaching vertically to the surface plane would be electronically Received: May 4, 2012 Revised: September 11, 2012 Published: September 25, 2012 21771

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favored.29,30 With the help of first-principles based calculations, Liu et al. pointed out the role of surface Ti atoms as reaction centers for the oxygen adsorption and dissociation.31 In a recent detailed study, they also proved the nature of O2 dissociative adsorption, evaluated the oxygen activation processes at the NiTi alloy surface, and suggested the important role of surface Ti atoms in oxygen activation.32 Till now, the atomic level processes, such as atomic adsorption and diffusion, that occur in the early stages of oxidation and is necessary for optimization of NiTi alloy processing techniques, remains a mystery. Nolan et al. studied the successive O2 adsorption on the NiTi(110) surface, and they highlighted the oxygen adsorption induced formation of the TiOx layer.33,34 Nigusaa et al. also studied the O adsorption on Ti-terminated (100) and (110) of B2-NiTi alloy, and they showed that O adsorption on 4-fold hollow site and bridge site is preferred.35,36 Though these works put emphasis on various aspects of the early stage oxidation of NiTi alloy, the temperature controlled formation of surface TiOx phase has never been mentioned. Furthermore, the evolution of adsorption structure with respect to O coverage and the detailed formation mechanism of the oxide layer at the alloy surface is also vital to explore the long-term biocompatibility issues in the application of NiTi alloys. To address this topic further, by first-principles based calculations, we provide a microscopic mechanism for the temperature controlled oxidation of NiTi alloy. We show that the oxygen atomic adsorption and diffusion on the NiTi surface are the elementary steps for the formation of a surface oxide layer. The oxygen will bind strongly with the surface and even induce surface reconstruction. The subsequent surface diffusion is thermodynamics driven, but the corresponding kinetics is strongly affected by temperature. These result in formation of TiO at low temperatures and TiO2 at elevated temperatures.

EO/NiTi, ENiTi, and 1/2EO2, respectively, while n is the number of O atoms in the slab. NiTi alloy can exist in two stable phases, namely, the Austenitic phase (B2) and the Martensitic phase (B19).46,47 Most of the reported electronic studies concerning NiTi alloy were dedicated to studying the phase transformation between the Austenitic phase and the Martensitic phase. It has been proved experimentally that the B2 phase is retained even after oxidation at 500 K.11,20,21 Therefore, the work involved in this paper will be focused on the B2-NiTi phase. The calculated lattice parameters are shown in Table 1. The accordance between calculated values and reference results proves the reliability of the approach used in this work. Table 1. The Calculated Bulk Modulus and Crystal Parameter of B2 NiTi Alloy bulk modulus (GPa)

crystal parameters (Å)

GGA-PBE (this work) GGA-PW91 (this work) GGA-revPBE (this work) LAPW48 TB-LMTO47 FP-LAPW49 FP-LAPW50 experimental51

149.9 153.6 150.9

3.015 3.012 3.022 3.016 3.031 3.01 3.01 3.015

170.1 157.8 140.3

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RESULTS AND DISCUSSION O Atomic Adsorption on NiTi(100) Surface at 1/4 ML. Extensive first-principles calculations were employed to explore all the possible O atomic adsorption structures on the NiTi(100) surface at 1/4 ML coverage. For B2 phase NiTi alloy, the high symmetry adsorption sites are atop (AT), bridge (BG), and the 4-fold hollow (4F) sites (Figure 1). Another adsorption site, where O absorbs in the middle of the triangle comprised by Ti1, Ti2, and Ti3, is also considered, and the optimization result shows that surface reconstruction would be induced to form stable Ti−O interaction and a 3-fold hollow (3F) site can be formed (Figure 1). The atomic structures of these adsorption structures and atomic nominations are shown in Figure 1. The adsorption structural and bonding details are presented in Table 2. It is apparent from Table 2 that the Eads values calculated with different functions at the same site vary slightly with functional and the structures do not differ significantly, which implies that all the calculations converge at the same ground state despite the choice of functional. Due to the formation of Ti−O bonds, the calculated Eads are at the order of −6.0 eV, which is in accordance with previous results of Liu et al. and Nigussa et al. on O2 adsorption on the NiTi alloy surface.31,35 The 3F is identified for the first time and has the highest stability, as the corresponding Eads is −6.14 eV. The AT is most unstable and is 2.55 eV less stable than that of 3F. The 4F and BG structures are also stable, and the differences in Eads among 3F, 4F, and BG sites are within 1.2 eV. With the Eads values shown in Table 2, the relative stability order among these adsorption sites is 3F > 4F > BG > AT. The Eads values calculated with different functionals are the same in the order of stability, proving the reliability of the calculation. The O adsorption on the eight-layer slab and nine-layer slab models were also investigated, and the calculated Eads values at the 3F

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COMPUTATIONAL DETAILS DFT calculations were performed by using the DACAPO code,37 where the ionic cores were described by ultrasoft pseudopotentials.38 The Kohn−Sham one-electron valence states were expanded in a plane-wave basis set with a kinetic energy cutoff at 340 eV, with the exchange and correlation energies approximated by the GGA-PBE functional.39,40 Most of the results are reproduced with the GGA-PW91 functional and the GGA-revPBE functional for justification.37,41 The NiTi(100) surface was represented by a seven-layer slab. Adsorption was allowed on the relaxed side of the slab, and the effect of the induced dipole moment was taken into account.42 The O atoms, the top four metal layers of NiTi(100), were fully optimized until the maximum residue force was below 0.02 eV/ Å. Supercells with a periodicity of (2 × 2) were employed to study adsorption at an initial coverage of 1/4 ML with respect to the oxygen atom. Also, a Monkhorst−Pack mesh with a (6 × 6 × 1) grid was used for integration in the reciprocal space.43 Moreover, the energy convergence with respect to the k point mesh was tested, and the adsorption energy difference with a (8 × 8 × 1) grid was less than 0.02 eV. The nudged elastic band (NEB) method and the modified NEB method (ANEBA) were used to locate the transition-state (TS) structure.44,45 The adsorption energy, Eads, was calculated as Eads(O) = [EO/NiTi − (E NiTi + n × 1/2EO2)]/n

calculation method and functional

(1)

where the total energy of the adsorbate−substrate system, the clean surface (NiTi(100)), and the O atom are represented by 21772

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Figure 1. Top view (upper panel) and side view (lower panel) of the adsorption structures of an O atom on the NiTi(100) surface, namely, the atop site (AT), the bridge site (BG), the 3-fold site (3F), and the 4-fold site (4F) (light blue, Ti atom; gray, Ni atom).

Table 2. The Optimized Structures and Adsorption Energies on NiTi(100) Surface AT adsorption site

a

R(O−Ti1)b/B(O−Ti1)c R(O−Ti2)/B(O−Ti2) R(O−Ti3)/B(O−Ti3) R(O−Ti4)/B(O−Ti4) h(O‑Surface)d R(Ti1−Ni1)/B(Ti1−Ni1) R(Ti1−Ni2)/B(Ti1−Ni2) R(Ti1−Ni5)/B(Ti1−Ni5) R(Ti1−Ti2) R(Ti1−Ti3) R(Ti3−Ti4) Eadse

BG

3F

4F

PBE

PBE

PBE

PW91

revPBE

PBE

PW91

1.67/0.92 3.48 /3.48 /4.56 /1.71 2.69/0.06 2.69/0.06 2.69/0.06 3.02 3.02 3.02 −3.59

1.84/0.50 3.83/1.84/0.50 3.83/1.47 2.56/0.15 2.56/0.15 2.52/0.14 3.08 2.79 3.08 −4.96

1.94/0.33 1.96/0.36 1.96/0.36 3.82/0.92 2.38/0.09 2.58/0.21 3.82/2.90 2.90 3.37 −6.14

1.93/0.33 1.96/0.36 1.95/0.36 3.81/0.92 2.38/0.09 2.57/0.20 3.81/2.89 2.89 3.38 −6.24

1.95/0.33 1.97/0.36 1.96/0.36 3.83 0.94 2.39/0.09 2.60/0.21 3.84/2.91 2.91 3.40 −6.08

2.08/0.28 2.10/0.28 2.11/0.26 2.08/0.28 0.66 2.53/0.23 2.57/0.21 2.67/0.03 2.81 2.83 2.83 −5.74

2.07/0.28 2.10/0.27 2.10/0.27 2.09/0.28 0.65 2.52/0.22 2.56/0.21 2.68/0.04 2.82 2.82 2.83 −5.79

a

See Figure 1 for notation of Ni and Ti atoms. bThe interatomic distance (R) between specific Ni and Ti atoms is presented as R(Ni−Ti) and is in units of Å. cThe bond order (B) is adapted to present the interaction between specific Ni and Ti atoms and is denoted as B(Ni−Ti). dThe distance between the O atom and the surface is denoted as h and is in units of Å. eEads is calculated as the difference between the energy of optimized structures and the summation of the energy of the corresponding clean surface and absorbed oxygen atoms in terms of 1/2 O2 following eq 1 and is in units of eV.

calculated B(O−Ti2) and B(O−Ti3) are all 0.36, and B(O− Ti1) is 0.30. The calculated ∠Ti1−O−Ti2, ∠Ti1−O−Ti3, and ∠Ti2−O−Ti3 bonding angles are 99.85, 99.85, and 105.92°, respectively. It should also be noted that, in the optimized adsorption structure, R(Ti1−Ti3) and R(Ti1−Ti2) are decreased from 3.02 to 2.90 Å, and R(Ti3−Ti4) is increased from 3.02 to 3.37 Å. In this structure, the original cubic symmetry of the surface Ti atoms is distorted to form a quasihexagonal structure. The structure change of this type can be taken as an adsorption induced surface reconstruction. As for the 4F structure, a quadrangular pyramid is formed by the O atom and four adjacent surface Ti atoms at the 4F site, where the O atom occupies the top corner and the top-bottom distance in the quadrangular pyramid (the height of the quadrangular pyramid) is 0.66 Å. There is no significant bond order and bond angle difference among the four formed O−Ti bonds. Differential charge analysis was conducted to illustrate the charge transfer and change in bonding induced by O adsorption, and the result is shown in Figure 2. For all the adsorption sites considered, the charge accumulation regions are mainly surrounding the O atoms and the charge transfer can

and 4F sites vary within 0.05 eV with the change of slab thickness and slab bottom termination, showing the calculated Eads is less dependent on the slab thickness for slab models thicker than seven layers. The bond order (B) analysis was performed by projecting of the wave function onto a localized basis set as a linear combination of basis sets of atomic orbitals. The result shows that B(O−Ti) is 0.92 for the AT, which implies the formation of typical chemical interaction between the O atom and the surface Ti atoms. At the same time, B(Ti1−Ni5) is significantly weakened to 0.06, showing the impact of O atomic adsorption on the local surrounding surface electronic structure. As for BG, the absorbed O atom is pulling the Ti1 and Ti3 together by forming successful O−Ti interaction. As a result, the calculated B(O−Ti1) and B(O−Ti3) are 0.5 and are equivalent, while the Ti1−Ti3 distance (R(Ti1−Ti3)) is decreased from 3.02 Å on the clean NiTi(100) surface to 2.79 Å. However, there is only a negligible change of R(Ti1−Ni5) observed, which indicates that the O adsorption will only locally impact the electronic structure of the surface Ti layer. For the most stable 3F site, both the structural and bond order analysis prove that the three formed O−Ti interactions are almost equivalent. The 21773

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Figure 2. Top view (upper panel) and side view (lower panel) of the contour plot of differential charge density for O adsorption on the NiTi(100) surface. The adsorption structures are the atop site (AT), the bridge site (BG), the 3-fold site (3F), and the 4-fold site (4F). The isovalue is ±0.001 au. The directions of the side views in the lower panel are indicated by the yellow short dashed line. The charge accumulation regions are shown in red , and the charge depletion regions are shown in blue (light blue, Ti atom; gray, Ni atom).

be identified as from the surface Ti atoms to O atoms along the Ti−O directions. The change transfer (both accumulation and depletion) regions are only observed among surface atoms, showing that only the bonding among surface atoms is affected by the O adsorption. As the charge accumulation regions are localized around O atoms in the Ti−O direction, the formed Ti−O bonds should have an ionic component and the O atom would be negatively charged. Bader analysis was also carried out to quantitatively understand the charge transfer upon adsorption.52 The calculated Bader charge (q) for the topmost Ti atom is 3.29| e|, and that for the subsurface Ni atom is 11.20|e|. For the AT site, Δq(O) is 0.94|e| and Δq(Ti1) is −0.42|e|, showing that the O atom gains electrons from the nearest surface metal atoms. As for the BG case, similar to the case for the AT site, Δq(Ti1) and Δq(Ti3) are −0.33|e| and Δq(O) is 1.03|e|. Due to the relative larger distance from the O atom, q(Ti2) and q(Ti4) vary only slightly (Ti− (4 × 3F + 2 × BG + 2 × AT, Figure 4), and the calculated Eads value is −4.06 eV. To release the residue forces, the absorbed O atoms move downward below the surface Ti plane, suggesting that the O diffusion to the subsurface is ready to take place at this coverage. The evolution of the O adsorption structures from 4F with respect to O coverage was also investigated. Starting from the 4F, the second O atom can either absorb at BG sites connecting the 4F structures in the adjacent (2 × 2) supercell or take the remaining 4F sites. The calculated Eads of when a second O atom absorbs at the nearest 4F site forming a linear structure (2 × 4F, Figure 5) is −5.50 eV, which is 0.28 and 0.03 eV more stable than at the BG site and the second nearest 4F site. This may be due to the fact that formation of the linear structure is preferable for release of the residue forces generated by the formation of the O−Ti bonds. The third O atom also prefers the 4F sites (3 × 4F, Figure 5), and as more O atoms gain charge from the surface, the adsorption is weakened and Eads decreases to −5.37 eV. When four O atoms take all four available 4F sites (4 × 4F, Figure 5), the coverage reaches 1 ML and the corresponding Eads is −5.21 eV. At the O coverage of 5/4 ML, there are three possible sites for adsorption of the addition O atom, namely, the surface bridge site (BG + 4 × 4F, Figure 5), the subsurface bridge site (sBG + 4 × 4F, Figure 5), and the atop site (AT + 4 × 4F, Figure 5). sBG + 4 × 4F is thermodynamically preferred, and Eads is −4.85 eV, which is 0.18 and 0.12 eV more stable than BG + 4 × 4F and AT + 4 × 4F, respectively. The high stability of the subsurface adsorption implies that the O diffusion to the subsurface may take place right after 1 ML coverage is reached. It should also be noted that, as for O coverage at 2 ML, O adsorption on the top surface (4 × AT + 4 × 4F) is preferred and the calculated Eads is −3.73 eV. O atoms prefer to stay as far apart as possible from each other and take the low coordinate sites to avoid interacting with each other. This finding is in accord with previous results by Nigussa et al.35 Comparing the evolution of the O adsorption energy with respect to the O coverage, it is possible to see that, no matter if you are starting from 3F or 4F, Eads decreases linearly with the O coverage. O prefers to bind the surface via 3-fold coordination with surface Ti atoms, though at some coverage the difference in stability as compared with those on 4-fold structure of the same coverage is quite small. O adsorption at AT and BG sites is only found stable when the high coordinate surface sites are all occupied. Both the Bader analysis and the differential charge density analysis support that O atoms gain electrons from the surface Ti atoms and generate dipoles at the 21776

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Table 4. Diffusion Rate Constant at 300 Ka

Table 3. Ea for the Forward Diffusion (Ea,forward) and Reverse Diffusion (Ea,reverse) on the NiTi(100) Surface diffusion path

Ea,forward (eV)

Ea,reverse (eV)

AT-BG AT-3F BG-4F BG-3F 4F-3F

0.022 0.004 0.140 0.079 0.263

1.458 2.177 0.475 0.824 0.673

diffusion path

k/k0

AT-BG AT-3F BG-4F BG-3F 4F-3F

0.505 1.000 0.006 0.058 5.366 × 10−5

The diffusion rate constant is calculated as k = (kBT/h)(q#/q)e−Ea/kBT, where h is the Plank constant, kB is the Boltzmann constant, ΔE is the reaction barrier, and q# and q are the partition functions of the transition state and the reactant. We found that the partition function is mainly contributed by the vibration of adsorbed O atoms, and therefore, the partition function can be simplified to the vibrational partition function, which was calculated from the vibration frequency of O along the diffusion direction.57,58 a

high as 2.177 eV, which means that the reverse diffusion would be very hard to take place. Similar to this, the Ea for O diffusion from AT to BG is 0.022 eV and is 0.018 eV higher than that for O diffusion from AT to 3F, and the Ea for the reverse diffusion is 1.458 eV. This proves that the adsorption stability at BG is much higher than AT from the kinetic aspect. Ea for the forward diffusion from BG to 3F is 0.079 eV, and that of the reverse process is 0.824 eV. The difference in activation energy is 0.743 eV, which means adsorption at 3F would be more stable than the BG, and this is in accord with the Eads analysis. As for the diffusion from BG to 4F, Ea is 0.140 eV, which is 0.335 eV lower than that of the reverse diffusion. The small values for Ea of the forward diffusion and the large value for the reverse diffusion indicate that, under normal conditions, only the forward diffusion can take place. Therefore, these Ea values also kinetically prove the stability difference among these adsorption sites. A full potential energy surface for O adsorption and diffusion on the NiTi(100) surface is also depicted in Figure 6.

However, as the rate constant for diffusion from the 4F site to the 3F site is the lowest, the diffusion of O atom from the 4F site to the 3F site would also be very slow even at 300 K. This implies that, if an O atom is adsorbed at the 4F site, it would not diffuse to anywhere else. These should hold for the submonolayer and monolayer adsorption, until all the available 4F sites and potential 3F sites are occupied. This suggests that the surface diffusion can be controlled by the processing temperature. On the NiTi(100) surface, O2 dissociation is a spontaneous process without barrier.31,32 Considering if there are more O atoms continuously absorbing onto the surface, the O atoms will take the remaining BG sites and AT sites, as indicated by results on the evolution of adsorption structures with respect to O coverage, and finally terminate the surface as a surface TiOx layer. Therefore, it can be expected that, at low temperatures (i.e., 300 K), as the diffusion from the 4F site to the 3F site is almost 0, the formation of 3F structure and 4F structure will be competing with each other. In this sense, the formed adsorption structure will be retained throughout the low temperature oxidation treatment. On the other hand, at high temperatures, the atomic diffusion will be accelerated, and the formed 4F structures can evolve to more stable 3F structures easily. To this end, it can be concluded that the oxidation treatment temperatures will have a dominant effect on the quality of the TiOx layer on the NiTi surface and well-defined processing conditions would be necessary to generate a condensed and uniform TiOx surface layer. This also explains the experimental findings for formation of a merely TiO phase during low temperature oxidation treatment, oxide mixture at moderate temperatures, and only rutile TiO2 phase at high temperatures.11,19 The lattice parameters of rutile TiO2(110) are a = 6.497 Å and c = 2.959 Å, and that of rocksalt TiO is a = 2.953 Å.55,56 The lattice mismatch of rutile TiO2(110) with respect to NiTi(100) was estimated to be 7.18% in the a-direction and 1.89% in the c-direction, and that of TiO(100) was calculated to be 2.10%. Though the formation of the TiO2 layer is thermodynamically preferred, defects will be generated in the TiO2 layer to release the resident forces at the interface and act as diffusion channels for the outward diffusion of Ni from the bulk. In this sense, lowering the processing temperature would be a feasible way to generate a condensed TiOx passive layer with a low ratio of defects.

Figure 6. Schematic view of the adsorption and diffusion potential energy surface of an O atom on the NiTi(100) surface with respect to the total energy of 3F.

With the Ea values in Table 3, according to the TST theory and the Arrhenius equation, the diffusion rate constant at 300 K is also calculated and listed in Table 4.52,53 Here, the arbitrary value of the diffusion rate constant for the AT-3F diffusion is taken as a unit. It is apparent that adsorption at 3F is of the dormitory superiority, as the diffusion rate constant from the other sites to 3F sites is the highest, and that of the 4F sites ranks as second. As for the high stability of the 3F and its position on the diffusion path from AT to 4F, the rate constant for AT-4F diffusion is not calculated directly. However, according to the PES shown in Figure 6, the corresponding Ea should be in the range of Ea along AT-BG and the AT-3F path at the most. Thus, the relative rate constant should be almost 1. This means that, when O first adsorbs onto the NiTi surface, if the surface reconstruction for formation of the 3F site is allowed, it will diffuse and generate a 3F adsorption structure. If formation of the 3F site is not allowed, it will diffuse to a 4F site. As the diffusion barrier from both the 4F site and 3F site is much higher, the outside diffusion from 3F can be omitted. 21777

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(7) Trigwell, S.; Hayden, R. D.; Nelson, K. F.; Selvaduray, G. Surf. Interface Anal. 1998, 26, 483−489. (8) Yeung, K. W. K.; Poon, R. W. Y.; Liu, X. Y.; Ho, J. P. Y.; Chung, C. Y.; Chu, P. K.; Lu, W. W.; Chan, D.; Cheung, K. M. C. J. Biomed. Mater. Res., Part A 2005, 72A, 238−245. (9) Yeung, K. W. K.; Poon, R. W. Y.; Liu, X. Y.; Ho, J. P. Y.; Chung, C. Y.; Chu, P. K.; Lu, W. W.; Chan, D.; Cheung, K. M. C. J. Biomed. Mater. Res., Part A 2005, 75A, 256−267. (10) Chu, C. L.; Wang, R. M.; Hu, T.; Yin, L. H.; Pu, Y. P.; Lin, P. H.; Wu, S. L.; Chung, C. Y.; Yeung, K. W. K.; Chu, P. K. Mater. Sci. Eng., C 2008, 28, 1430−1434. (11) Firstov, G. S.; Vitchev, R. G.; Kumar, H.; Blanpain, B.; Van Humbeeck, J. Biomaterials 2002, 23, 4863−4871. (12) Barison, S.; Cattarin, S.; Daolio, S.; Musiani, M.; Tuissi, A. Electrochim. Acta 2004, 50, 11−18. (13) Tollefsen, H.; Raaen, S. J. Appl. Phys. 2009, 105, 123501. (14) Wang, R. M.; Chu, C. L.; Hu, T.; Dong, Y. S.; Guo, C.; Sheng, X. B.; Lin, P. H.; Chung, C. Y.; Chu, P. K. Appl. Surf. Sci. 2007, 253, 8507−8512. (15) Pohl, M.; Glogowski, T.; Kuhn, S.; Hessing, C.; Unterumsberger, F. Mater. Sci. Eng., A 2008, 481, 123−126. (16) Lutz, J.; Lindner, J. K. N.; Mandl, S. Appl. Surf. Sci. 2008, 255, 1107−1109. (17) Wong, M. H.; Cheng, F. T.; Pang, G. K. H.; Man, H. C. Mater. Sci. Eng., A 2007, 448, 97−103. (18) Lelatko, J.; Goryczka, T.; Paczkowski, P.; Wierzchon, T.; Morawiec, H. J. Microsc. (Oxford, U.K.) 2010, 237, 435−438. (19) Xu, C. H.; Ma, X. Q.; Shi, S. Q.; Woo, C. H. Mater. Sci. Eng., A 2004, 371, 45−50. (20) Shabalovskaya, S. A.; Tian, H.; Anderegg, J. W.; Schryvers, D. U.; Carroll, W. U.; Van Humbeeck, J. Biomaterials 2009, 30, 468−477. (21) Vojtech, D.; Joska, L.; Leitner, J. Appl. Surf. Sci. 2008, 254, 5664−5669. (22) Huang, X. Y.; Ackland, G. J.; Rabe, K. M. Nat. Mater. 2003, 2, 307−311. (23) Canto, G.; de Coss, R. Surf. Sci. 2000, 465, 59−64. (24) Canto, G.; de Coss, R.; Papaconstantopoulos, D. A. Surf. Rev. Lett. 1999, 6, 719−723. (25) Kulkova, S. E.; Egorushkin, V. E.; Eremeev, S. V.; Kim, J. S.; Lee, G.; Koo, Y. M. Physica B 2004, 349, 342−347. (26) Koroteev, Y. M.; Lipnitskii, A. G.; Chulkov, E. V.; Silkin, V. M. Surf. Sci. 2002, 507, 199−206. (27) Kulkova, S. E.; Egorushkin, V. E.; Eremeev, S. V.; Kulkov, S. S. Mater. Sci. Eng., A 2006, 438, 476−479. (28) Kulkova, S. E.; Valujsky, D. V.; Kim, J. S.; Lee, G.; Koo, Y. M. Phys. Rev. B 2002, 65, 085410. (29) Hua, Y. J.; Liu, X.; Meng, C. G.; Yang, D. Z. J. Mater. Sci. Technol. 2004, 20, 182−185. (30) Hua, Y. J.; Liu, X.; Meng, C. G.; Yang, D. Z. J. Wuhan Univ. Technol., Mater. Sci. Ed. 2003, 18, 6−10. (31) Liu, X.; Meng, C. G.; Liu, C. H. Acta Metall. Sin. 2006, 42, 421− 425. (32) Liu, X.; Guo, H.; Meng, C. Mater. Sci. Forum 2011, 675−677, 353−356. (33) Nolan, M.; Tofail, S. A. M. Biomaterials 2010, 31, 3439−3448. (34) Nolan, M.; Tofail, S. A. M. Phys. Chem. Chem. Phys. 2010, 12, 9742−9750. (35) Nigussa, K. N.; Stovneng, J. A. Phys. Rev. B 2010, 82, 245401. (36) Nigussa, K. N.; Stovneng, J. A. Comput. Phys. Commun. 2011, 182, 1979−1983. (37) Hammer, B.; Hansen, L. B.; Norskov, J. K. Phys. Rev. B 1999, 59, 7413−7421. (38) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892−7895. (39) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1993, 48, 4978− 4978. (40) White, J. A.; Bird, D. M. Phys. Rev. B 1994, 50, 4954−4957. (41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868.

CONCLUSIONS With the help of first-principles based calculations, the TiOx species formation mechanism on the NiTi surface was investigated. Extensive calculations were carried out to study the oxygen adsorption and diffusion on the NiTi alloy (100) surface. The O atom will bind strongly with the surface, and even induce surface reconstruction. Four typical structures will be formed at the initial, namely, atop (AT), bridge (BG), 3-fold hollow (3F), and 4-fold hollow (4F) sites. Also, the relative stability of these structures is 3F > 4F > BG > AT. The further evolution of the surface adsorption structures with O coverage was also investigated, and results show that both TiO- and TiO2-like surface structures can be formed. The diffusion of O atom is mainly from the AT site and BG site. Due to the requirement of strong surface reconstruction for formation of the 3F structure, the diffusion from the 4F site to the 3F hollow site is relatively slow, and the corresponding kinetics will be strongly affected by the handling temperatures. In this sense, the O atomic adsorption and diffusion on the NiTi surface are the elementary steps for the formation of a surface oxide layer. These findings also help to explain the controlling effect of handling temperature on the formation of a surface TiOx layer: TiO would be the main TiOx phase at low oxidation temperature, rocksalt TiO phase and rutile TiO2 phase will coexist at moderate temperatures, and rutile TiO2 phase is the sole TiOx phase at high temperature. The current work provides new insights on the initial oxidation of the NiTi surface, and would be beneficial for design of well-defined processing technologies (e.g., controlled handling temperature, etc.) to generate a condensed passivation layer to improve the biocompatibility of NiTi alloy based biomaterials.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-411-84708545. Email: [email protected] (X.L.); [email protected] (H.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21103015, 11174045, and 20273012), the Chinese Scholarship Council (2009606533), the Young Teacher Training Program of Dalian University of Technology (1000-893238, 1000-893374), the Fundamental Research Funds for the Central Universities (DUT11LK19, DUT12LK14), and the Key Laboratory of Coastal Zone Environmental Processes YICCAS (201203) for financial support.

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REFERENCES

(1) Duerig, T.; Pelton, A.; Stockel, D. Mater. Sci. Eng., A 1999, 273, 149−160. (2) Wever, D. J.; Veldhuizen, A. G.; Sanders, M. M.; Schakenraad, J. M.; van Horn, J. R. Biomaterials 1997, 18, 1115−1120. (3) Shabalovskaya, S.; Anderegg, J.; Van Humbeeck, J. Acta Biomater. 2008, 4, 447−467. (4) Zhang, L.; Xie, C. Y.; Wu, J. S. Mater. Charact. 2007, 58, 471− 478. (5) O’Brien, B.; Carroll, W. M.; Kelly, M. J. Biomaterials 2002, 23, 1739−1748. (6) Gu, Y. W.; Tay, B. Y.; Lim, C. S.; Yong, M. S. Appl. Surf. Sci. 2005, 252, 2038−2049. 21778

dx.doi.org/10.1021/jp304343k | J. Phys. Chem. C 2012, 116, 21771−21779

The Journal of Physical Chemistry C

Article

(42) Bengtsson, L. Phys. Rev. B 1999, 59, 12301−12304. (43) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192. (44) Sheppard, D.; Terrell, R.; Henkelman, G. J. Chem. Phys. 2008, 128. (45) Maragakis, P.; Andreev, S. A.; Brumer, Y.; Reichman, D. R.; Kaxiras, E. J. Chem. Phys. 2002, 117, 4651−4658. (46) Wang, F. E.; Buehler, W. J.; Pickart, S. J. J. Appl. Phys. 1965, 36, 3232−3239. (47) Cai, J.; Wang, D. S.; Liu, S. J.; Duan, S. Q.; Ma, B. K. Phys. Rev. B 1999, 60, 15691−15698. (48) Rhee, J. Y.; Harmon, B. N.; Lynch, D. W. Phys. Rev. B 1996, 54, 17385−17391. (49) Kellou, A.; Nabi, Z.; Tadjer, A.; Amrane, N.; Fenineche, N.; Aourag, H. Phys. Status Solidi B 2003, 239, 389−398. (50) Bihlmayer, G.; Eibler, R.; Neckel, A. Phys. Rev. B 1994, 50, 13113−13117. (51) Mercier, O.; Melton, K. N.; Gremaud, G.; Hagi, J. J. Appl. Phys. 1980, 51, 1833−1834. (52) Bader, R. F. W. Chem. Rev. 1991, 91, 893−928. (53) Li, W. X.; Stampfl, C.; Scheffler, M. Phys. Rev. B 2002, 65, 075407. (54) Li, W. X.; Stampfl, C.; Scheffler, M. Phys. Rev. B 2003, 67, 045408. (55) Diebold, U. Surf. Sci. Rep. 2003, 48, 53−229. (56) Banus, M. D.; Reed, T. B.; Strauss, A. J. Phys. Rev. B 1972, 5, 2775−2784. (57) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Norskov, J. K. Science 2005, 307, 555−558. (58) Chorkendorff, L.; Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics; WILEY-VCH Verlag GmbH & Co.: KGaA, Weinheim, Germany, 2006.

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dx.doi.org/10.1021/jp304343k | J. Phys. Chem. C 2012, 116, 21771−21779